In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise at least one imageable layer applied over the hydrophilic surface of a substrate. The imageable layer(s) include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged (exposed) regions are removed, the element is considered as positive-working. Conversely, if the non-imaged (non-exposed) regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or a fountain solution and repel ink.
Direct digital imaging has become increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Thermally imageable, multi-layer positive-working elements are described, for example, in U.S. Pat. No. 6,294,311 (Shimazu et al.), U.S. Pat. No. 6,352,812 (Shimazu et al.), U.S. Pat. No. 6,593,055 (Shimazu et al.), U.S. Pat. No. 6,352,811 (Patel et al.), and U.S. Pat. No. 6,528,228 (Savariar-Hauck et al.), and U.S. Patent Application Publication 2004/0067432 A1 (Kitson et al.). U.S. Patent Application Publication 2005/0037280 (Loccufier et al.) describes heat-sensitive printing plate precursors that comprise a phenolic developer-soluble polymer and an infrared radiation absorbing agent in the same layer.
Additional positive-working thermally imageable elements are described and used for making lithographic printing plates using various developers in U.S. Pat. No. 6,200,727 (Urano et al.), U.S. Pat. No. 6,358,669 (Savariar-Hauck et al), and U.S. Pat. No. 6,534,238 (Savariar-Hauck et al.). In some instances, such imageable elements are developed using low pH developers when the upper layer includes novolak resins and dissolution suppressing agents.
Single-layer, positive-working imageable elements are described for example, in U.S. Pat. No. 6,280,899 (Hoare et al.), U.S. Pat. No. 6,391,524 (Yates et al.), U.S. Pat. No. 6,485,890 (Hoare et al.), U.S. Pat. No. 6,558,869 (Hearson et al.), and U.S. Pat. No. 6,706,466 (Parsons et al.), and U.S. Patent Application Publication 2006/0130689 (Müller et al.).
Copending and commonly assigned, U.S. Ser. No. 11/686,981 (filed Mar. 16, 2006 by Savariar-Hauck et al.) describes and claims a method of processing positive-working imageable elements to prepare lithographic printing plates. Other imageable elements are described in U.S. Pat. No. 6,555,291 (Savariar-Hauck).
Development of negative-working elements using gums is described for example, in EP Publications 1,751,625 (Van Damme et al. published as WO 2005/111727) U.S. Pat. No. 1,788,429 (Loccufier et al. et al.), U.S. Pat. No. 1,788,430 (Williamson et al.), U.S. Pat. No. 1,788,431 (Van Damme et al.), U.S. Pat. No. 1,788,434 (Van Damme et al.), U.S. Pat. No. 1,788,441 (Van Damme), U.S. Pat. NO. 1,788,442 (Van Damme), U.S. Pat. No. 1,788,443 (Van Damme), U.S. Pat. No. 1,788,444 (Van Damme), and U.S. Pat. No. 1,788,450 (Van Damme), and WO 2007/057442 (Gries et al.). In addition, copending and commonly assigned U.S. Ser. No. 11/872,772 that was filed Oct. 16, 2007 by K. Ray, Tao, Miller, Clark, and Roth) describes negative-working imageable elements that are sensitive to infrared radiation and can be processed using gum solutions.
Copending and commonly assigned U.S. Ser. No. 11/947,817 (filed Dec. 4, 2007 by K. Ray, Tao, and Clark) describes the use of gums to develop imaged UV-sensitive, negative-working imageable elements that contain specific nonpolymeric diamide additives.
Problem to be Solved
There is a fundamental difference between the imaged regions in negative-working and positive-working imageable elements. In negative-working imageable elements, the exposed (imaged) coating regions are crosslinked or coalesced and are generally insoluble in processing fluids. However, the non-exposed coating(s) regions remaining in positive-working imageable elements can be removed if a processing solution is left on it for an extended period of time. This fundamental difference between the chemistries used in the two types of coatings is particularly evident when the positive-working imageable elements are composed of IR-sensitive chemistries rather than the older naphthoquinonediazide chemistry. The difference in rate of dissolution between exposed and non-exposed IR-sensitive coatings is relatively small. Thus, a user needs to develop or process such coatings and remove residual processing solution as soon as possible.
While known positive-working imageable elements have been commercialized to great success, they are generally processed (developed) using various high alkaline developers. A separate finishing gum is normally applied to the developed image to protect the plate during handling. There is a desire to process such elements in a simplified process using more environmentally acceptable processing solutions without the loss in the desired imaging properties. While negative-working imageable elements have been processed with gum solutions, until now, it was not expected that imaged positive-working imageable elements, containing very different chemistries and multiple layers, could be similarly processed and protected before printing because of the fundamental differences in imaging chemistries noted above.